Devonian Era in Ohio Basin. — L'laypole. 341 
complete solution in the water. Their lime would in that case 
have afforded material for another generation and no trace of 
their past existence would remain. As in confirmation of this 
opinion, I have often found in the Chemung strata of Penn- 
sylvania the casts of single valves of brachiopods and lamelli- 
branchs suggesting that the fossil was the impression rather 
of an epidermis of a solid shell, the latter having apparently 
been dissolved away before fossilization took place. Only oc- 
casionally in such circumstances would the preservation of the 
organic remains of the time be possible. 
The abundance of pyrites diffused through the mass is an- 
other reason for believing that organic remains are not lack- 
ing in the Black shale in consequence of original absence. In 
some cases this mineral is so abundant as to earn for the beds 
the name of alum-shales or schist. This fact also lends sup- 
port to the belief that the accumulation was very slow, inas- 
much as the iron sulphide is regarded as one of the derivatives 
of animal decay. 
That fishbones should be preserved in so great perfection 
in the almost total absence of all other fossils is again a fact 
in the same line. These parts, consisting chiefly of phosphate 
of lime are not subject to the same laws of solution as are 
the shells of Mollusca, composed of the carbonate. A further 
fact tending in the same direction is that almost the only mol- 
luscan remains in these olack shales are those of the genera 
Lingula and Discina, with their allies. These contain little 
or no lime, and could, like the fish plates, resist the solvent 
powers of the water and endure until they were buried in sedi- 
ment. The excessive abundance of these horny shells in cer- 
tain layers renders it difficult to believe in the total absence of 
every other kind. Mud-loving lamellibranchs are far from 
scarce. 
We cannot enter into the mixed chemical question of the 
exact methods of production of petroleum and the other hydro- 
carbons found in like situations. They are apparently in the 
main, similar to those which develop marsh-gas when vege- 
table matter is buried under water, without decaying. But the 
nature of the petroleum is frequently in accord with thai of the 
shale, the black heavy oil being the product of the Black shaje 
while ihe lighter and redder oils of Pennsylvania, rich in kero- 
